In this application we want to resolve some of the molecular and cellular interactions that lead to the formation of the two main sensorineuronal cell types of the ear, the hair cell for mechanoelectric transduction and the sensory neurons for conduction of information from the hair cells to the brain. Using morphological (e.g. immunohistochemistry and DiI labeling) and molecular biological approaches (e.g. in situ hybridization, and RTPCR) we will analyze aspects of 1) cell survival, 2) general cell fate determination, and 3) organ specific cell fate determination. In AIM 1 we will scrutinize the role played by both location and amount of neurotrophin expression in patterning inner ear innervation. Inner ear sensory neurons express both trkB and trkC receptors. This unique feature will allow us to analyze in a BDNF null mutant simultaneously carrying a neurotrophin transgene (NT3 tgBDNF) for the first time in vivo how both concentration and distribution of a neurotrophin affect the patterning of normal and aberrant nerve fibers. In AIM 2 we will examine the molecular and cellular interactions of the principle neurogenic bHLH genes expressed in the ear; ngn1, Math1 and NeuroD. We will examine a possible clonal relationship between sensory neurons and hair cells and how the absence of varying combinations of proneuronal bHLH genes affects ear development, in particular the gene expression patterns in supporting cells. In contrast to the brain, this analysis is feasible because only two bHLH genes (Math1, ngn1) determine two neuron-like cells (hair cells, sensory neurons). We will interbreed mutants of ngn1, Math1, and NeuroD to study how ngn1 affects Math1, how ngn1 affects NeuroD, how Math1 affects NeuroD and how double nulls for both ngn1 and Math1 affect formation of the remaining ear. In AIM 3 we will analyze how BF1 and FGF10 affect the formation of organ specific sensory neurons and sensory epithelia absent in these mutants. We will characterize the expression of the bHLH genes, ngn1 and Math1, in null mutants of BF1 and FGF10. Such information can guide future research on regeneration of these cells in patients suffering sensorineuronal hearing loss and other areas of neuroscience less amenable to such investigations.